Field of the Invention
THIS INVENTION relates to shot pellets and the like.
A Many thousands of tonnes of lead shotgun pellets are scattered on the surface of the earth and embedded in trees each year in the act of vermin, game and clay target shooting for both pleasure and vermin control purposes It is now recognized that where this falls on wetlands it may be accidentally ingested by wildfowl together with their normal grit diet deliberately consumed as an essential part of their gizzard digestive process. The outcome is that the lead is ground up by the gizzard resulting in poisoning, debilitation and death. A further problem now recognized is that lead shot deposited on land where crops are grown can be dissolved and enter into the structure of the crop which is designed for human consumption.
A similar problem of wildfowl poisoning caused by the lead weights used by fishermen has been resolved by the adoption of alternative heavy materials for the weights. Attempts to apply a similar solution to the lead shot used in shotgun cartridges have proved much more difficult because of the stringent requirements imposed by the need for effective ballistics, safe performance and the economics related to the precious nature of many heavy metals. A shotgun shot must have the correct physical properties that allow it to provide correct ballistics and yet allow it to pass safely through a shotgun barrel at very high pressure without risking safety related to the proof of the gun.
One key property of lead that makes it so successful as a shot material is its high density, 11.35 tonnes per m.sup.3, because the energy associated with the shot at the moment it strikes the target relates to its mass and its velocity as E=1/2 mv.sup.2. A second property of lead is its softness allowing it to pass through a gun barrel safely and without causing damage to the barrel structure despite high pressure and velocity. A third property is the ability of lead spheres to flatten slightly and retain the flattened shape thereby showing no elastic tendency. This enables the energy contained within the mass of the sphere to be transferred to the target with maximum lethal effect.
Lead has a modest position in the list of abundances of the metallic elements at 10 parts per million and poses no problem of dwindling resource.
Iron has been proposed as an alternative and has found some use but its density is only 7.86 tonnes per m.sup.3 which means it only carries 69.25% of the striking energy provided by lead shot of the same size. Iron shot also offers problems because of its hardness and rigidity, which causes damage to the steel gun barrel bores of the modern shotgun, and has a tendency to create abnormally high and dangerous pressures. Iron based shot has a tendency to corrode so that the individual shot spheres bind together producing a dangerous solid slug which can destroy the gun barrel. Iron based shot can become embedded in growing timber and poses a dangerous threat to timber processing machinery and the elasticity of iron and steel results in shot that ricochets dangerously and does not transmit its energy to the target in an effective and lethal manner resulting in wounding of live targets.
Bismuth has also been proposed as an alternative and has found some use. The density is 9.747 tonnes per m.sup.3 and is approaching lead but its abundance is only 0.004 parts per million and it is a secondary metallurgical material being a by-product of the refining of other metals. The price is high and the source precarious which means any attempt to adopt it generally would result in prohibitive price escalation. Bismuth is a very brittle metal and can only be made more usable if it is alloyed with expensive tin or toxic lead. There are also unresolved questions about its toxicity when ingested by animals and humans.
It has been proposed to use tungsten, tungsten alloys or mixes of tungsten and other metals such as molybdenum as fillers in plastics matrices of various kinds as a basis for an alternative shot material, for example as exemplified in GB-A-2200976 and W094/24511. However, it has been found that known substitute shot materials are inferior to lead as a shot material in various respects. Indeed, some previous attempts to produce substitutes for lead shotgun shot have proved disastrous because the shot has either tended to shatter or has tended to stick together and has caused damage to gun barrels because the formulations used have included-polymers that cannot form a proper matrix and because the mixture used was abrasive.
Experiments conducted by the applicants suggest that, apart from density, the deformability of the shot material is also important. That is to say, that experiments suggest that both (a) resilience, i.e. recoverable deformability and (b) the ability to deform permanently (at least in the short term) on striking the target, are important. Thus, shot which is substantially rigid tends to lack "stopping power" for shooting game, in that such shot tends to pass through the game with significantly less transfer of energy to the game than would corresponding lead shot and thus tends to wound, rather than kill, the game. On the other hand, shot which is too readily deformable, and, in particular, which too readily undergoes a permanent deformation, produces too open a pattern at typical target distances. The applicants hypothesize that this is due to such shot becoming flattened by the acceleration imparted to it when the gun is fired, so that the individual shot are no longer spherical and are deflected slightly, and randomly, by aerodynamic forces. Furthermore, shot which is too readily flattened on striking a target again tends to wound, rather than kill, game because penetration is insufficient since, presumably, too much energy has been lost by excessive flattening of the shot and/or such energy has been spread over a larger frontal area of the target.
It is an object of the present invention to provide an improved alternative shot material without the disadvantages of toxicity, elasticity, brittleness and high price whilst possessing the qualities of high density, softness and an ability to transmit to target the striking energy resulting in effective lethality.
According to one aspect of the invention, there is provided shot for shotgun cartridges comprising finely divided metallic particles in a polymer matrix comprising:
(a) polypropylene or polypropylene copolymer and PA1 (b) a terpolymer of acrylic ester, ethylene and maleic anhydride.
According to another aspect of the invention there is provided shot for shotgun cartridges comprising finely divided metallic particles in a polymer matrix comprising ethylene propylene copolymer.
Said polymer mix may also include polyisobutylene.
Preferably said polymer matrix comprises a major proportion of polypropylene.
According to another aspect of the invention, there is provided a mouldable thermoplastic composition, suitable for making shot for shotgun cartridges, the composition comprising finely divided metallic particles in a polymer matrix comprising a blend of polymers.
Preferably, the metallic particles comprise materials selected from the group comprising iron, tungsten, molybdenum, alloys of tungsten or molybdenum with other metals or mixtures of such materials.
The metallic particles may comprise a mixture of tungsten or a tungsten alloy with molybdenum or a molybdenum alloy.
The blend of polymers preferably comprises a blend of a polyolefinic polymer, a styrene based polymer and a polymer containing maleic anhydride.
The polyolefinic polymer may be one selected from the group comprising LDPE (low density polyethylene), LLDPE (linear low density polyethylene), EVA (ethylene vinyl acetate copolymer), EEA (ethylene ethyl acrylate copolymer), ionomers (copolymers of alkenes and alkacrylic acids with metal ion crosslinks), polybutene, poly (4methylpent-1-ene), PP (polypropylene) homopolymer, or PP (polypropylene) copolymer, or mixtures thereof, said polyolefinic polymer comprising from 50% to 90% by weight of the composition.
The styrene-based polymer may be selected from the group comprising polystyrene, HIPS (high impact polystyrene), SAN (styrene acrylonitrile polymer), ABS (acrylonitrile butadiene styrene terpolymer) or a polystyrene/polyphenylene oxide blend, such as the blend of poly-2, 6-dimethyl-p-phenylene oxide and polystyrene or a polystyrene-related material, sold by General Electric under the Trade Mark NORYL.
According to another aspect of the invention there is provided a method of manufacturing shot for shotgun cartridges including mixing finely divided metal particles with a molten thermoplastics polymer, forming the resultant mixture into a plastics strip, web or strand, passing said strip, web or strand between two aligned rollers with cooperating hemispherical indentations to produce, on the exit side of said roller, a strip comprising a series of substantially spherical bodies connected and separated by a relatively thin web of the plastics material, subsequently placing said web between a first tool provided with apertures of a size to receive said spherical bodies and a second tool, such that said web overlies said first tool and said spherical bodies are seated in said apertures in the first tool, and pushing the spherical bodies through said recesses by means of said second tool, thereby punching the spherical bodies from the web with a minimum of equatorial "flash" or other discontinuities. Thus the action of said further rollers is to separate said spherical bodies from said web around peripheral lines of separation close to the peripheries of said spherical bodies. Said first and second tool may also comprise cooperating rollers, which may receive said strip from the first-mentioned rollers and may be driven in synchronism therewith.
The finely divided metallic particles may comprise tungsten, a mixture of molybdenum and tungsten, or a tungsten alloy such as ferro-tungsten, which has been found to have favorable properties, although it is of lower density than tungsten.
According to a still further aspect of the invention, there is provided a method of manufacturing shot for shotgun cartridges including mixing finely divided metallic particles in a polymer matrix comprising a blend of propylene or propylene polymers, to form a composition as described above forming the resultant mixture into a formable plastic web, passing said web between aligned rollers and thereby producing, directly or indirectly, substantially spherical bodies with equatorial flash therearound, and thereafter tumbling said bodies in a heated drum to remove such flash from the bodies.
The invention proposes a form of composite shot in which powdered metal, for example a mixture of powdered molybdenum and tungsten, is bound into a solid pellet by the use of polymeric materials. Preferably the material is present in just sufficient quantity to fill, or almost fill, the voids between the particles of the powdered metal such that the mix is close to the condition of close packing of spheres which means that about two thirds of the volume is metal powder. This, at 70% by volume in a binder matrix of unit density, molybdenum alone would give a pellet of density about 7.51 tonnes per m.sup.3. If only 23% of the metal in the mix is replaced by powdered tungsten then a pellet of density 8.42 tonnes per m.sup.3 is created which would have 13.63% more striking energy than an iron pellet and yet would be compliant because of the nature of the polymeric binder.
Alternatively, a powdered tungsten alloy, such as ferro-tungsten, may be used as the metal filler.
It is further proposed to include in the polymer/metal mix minor amounts of a lubricant substance such as molybdenum sulphide or graphite which would further improve the performance and minimize the wear of the gun barrels. Waxes and oils may be included in the mix to aid blending and flow in manufacture.
The preferred polymeric binder or matrix comprises (a) polypropylene or polypropylene copolymer (that is to say a copolymer of propylene and ethylene in which the ethylene content is relatively small, for example around 4%) and (b) a terpolymer of acrylic ester ethylene and maleic anhydrides. The component (b) may be the material supplied by Elf Atochem under the name "Lotarder".
A less preferred polymeric binder comprises a blend of ethylene propylene copolymer and polyisobutylene.
Embodiments of the invention are described below by way of example.